The present invention relates especially to a measuring device having a digital measurement transmitter, especially a measurement transmitter with an electrical current signal, in the case of which thus the measured value is output through control of a signal current, respectively a supply current. Digital measurement transmitters are those, which comprise at least one microprocessor for conditioning the measurement signals, respectively for controlling internal functions. A standard widely used in industry for delivering measurement signals of a physical and/or chemical, measured variable is an electrical current loop, e.g. a two conductor, electrical current loop. The measuring device is connected with a process controller via the electrical current loop. In such case, the measured value is represented by the value of the set electrical current and the electrical current is registered by the process controller. The electrical current transferred via the electrical current loop lies according to this widely used industrial standard between 0 mA and 20 mA.
Especially in safety-relevant applications, it is required to be able to detect the failure of a measurement transmitter, respectively its components, with sufficiently high probability. In the NAMUR recommendation NE43, it is provided, for example, that in the case of measuring devices a measurement signal current in a range between 4 and 20 mA is used. A device failure is signaled with a failure signal current outside this range, e.g. no greater than 3.6 mA, respectively at least 21 mA. In such case, an electrical current between 4 milliampère (mA) and 20 mA set in the electrical current loop corresponds to the measured physical and/or chemical, measured variable. Due to drift and inaccuracies, a somewhat larger electrical current range is permitted, for example, an electrical current between 3.8 mA and 20.5 mA. Electrical currents smaller than 3.6 mA, respectively greater than 21 mA, should not be interpreted by evaluating units as corresponding to a measured value. An electrical current set in the electrical current loop smaller than 3.6 mA or greater than 21 mA is, consequently, identified as an error current.
The analog measurement signal current set in the 2L electrical current loop is typically set by a control circuit in the 2L electrical current loop. In such case, at least one part of the measuring and operating electronics is part of the control circuit. For purposes of the control, a feedback variable corresponding to the actual value of the measurement signal current is read back by the measuring and operating electronics and compared with a reference input corresponding to the desired value. Usually, the reference variable is output by a control unit. The control unit outputs the reference variable corresponding to measurement signals of a measuring transducer or sensor. In the case of a measured value change, the actual value set in the 2L electrical current loop differs from the desired value. Produced from the control error signal between the actual value and the desired value is the corresponding manipulated variable, by means of which the measurement signal current in the 2L electrical current loop is changed.
A similar interface for the transmission of measurement signals and status information in digital form is standardized in EN 60947-5-6 (corresponding to IEC 60947-5-6). The operating parameters, thus the values of the electrical current associated with the digital states ON and OFF in NAMUR operation, differ from the above described. For the transmission of FPS NAMUR signals (Frequency/Pulse/Status) meeting EN 60947-5-6, the electrical current value for the ON state amounts to a value in the range 2.2 mA to 8 mA and the electrical current value for the OFF state lies in the range 0.35 mA to 1.2 mA in the case of a typical supply voltage of 8.2 V.
If the actual electrical current values lie under 0.35 mA, these can be evaluated as indication of a line interruption, while an electrical current value of over 8 mA can be evaluated as indication of a short circuit.
In the case of many applications, the size of the transmitted measurement signal can have a considerable influence on the surroundings of the measuring device and/or on the environment. Especially in the case of chemical and environment endangering processes, where a measured value of temperature, fill level, pressure, flow or composition of a measured material is transmitted, a safe running of the process is decisive. Therefore, it is extremely important that the transmitted measurement signal actually correspond to the measured value. Devices and systems in safety-relevant applications must, consequently, satisfy special requirements, among others fulfill the standards for functional safety (e.g. IEC 61508, IEC 61511, etc.). A central component of standards for functional safety is the so-called safety integrity level (SIL). Depending on the danger potentially emanating from a process or a plant, a corresponding SIL capability is required of an application. The SIL gives the probability of an arising failure of a device or an application being detected. In such case, the SIL ranges between 1 and 4, wherein 1 is the lowest and 4 the highest safety rating. Known from Offenlegungsschrift EP 1 860 513 is a circuit, which is intended to assure that an electrical current set in an electrical current loop corresponds to the actual value of the measured variable to be represented. The circuit relies on redundant adjusting of the value of the loop current and monitoring of the same. While the redundantly designed, electrical, electronic and/or electronically programmable components used in the state of the art in given cases increase the functional safety of the equipment, to be viewed as disadvantageous are the likewise increased manufacturing- and fabrication costs for a redundant design of the system and its components.
In German Publication, DE 102008001832 A1, it is provided that the evaluation unit compares at least one value of the manipulated variable output by the electrical current controller with at least one reference value and that the comparison yields whether the value of the manipulated variable exceeds or subceeds the reference value. The proposed evaluation unit enables detection of the deviation of the manipulated variable from a reference value or the exceeding or subceeding of the manipulated variable over or under a predetermined reference value. If especially two different reference values are provided, then it is detectable whether the manipulated variable exceeds or subceeds one of the reference values or lies in a region between the reference values. Since errors or disturbances of the measuring and operating electronics often appear in the form of an abnormal measurement signal, for example, in the form of excessive or lessened voltages and electrical currents, the described evaluation unit enables detection of such abnormal deviations.
Known from German Publication, DE 102004019392 A1 is a measurement transmitter, which includes a microprocessor with a reset input and a clock signal output for providing a periodic clock signal; further provided is a monitoring circuit with a clock signal input and a reset output and an electrical current controller for output of a measurement signal current, which measurement signal current represents in measurement operation in a first range a measured value and outside of the first range signals a failure; wherein the clock signal input of the monitoring circuit is connected with the clock signal output of the microprocessor, the reset input of the microprocessor is connected with the reset output of the monitoring circuit, in the case of failure of the clock signal a reset signal is periodically output on the reset output of the monitoring circuit, wherein, furthermore, the measurement transmitter has a comparator circuit with a first input, which is connected via a lowpass with the reset output of the monitoring circuit, with a second input, on which a reference voltage is applied, and with an output, which is connected with an input of the electrical current controller, wherein after repeated output of the reset signal the voltage on the first input of the comparator circuit exceeds the reference voltage, so that there is applied to the output of the comparator a control signal, which causes the electrical current controller to output a failure signal current outside of the first range. As indicated above, the first range for the measurement signal current amounts, for example, to 4 to 20 mA. In this case the failure signal current should be at least 21 mA or at most 3.6 mA, while preferably the failure signal current is controlled to 22 mA.
As already mentioned above, a similar interface for the transmission of measurement signals and status information is standardized in digital form in EN 60947-5-6 (corresponding to IEC 60947-5-6). The operating parameters, thus, the values of the electrical current in NAMUR operation associated with the digital states ON and OFF, differ from those above described. A simple and usual circuit of an EN 60947-5-6 switching amplifier for the transmission of FPS NAMUR signals (Frequency/Pulse/Status) includes a switch transistor with a parallel resistor across the transistor between the terminals of the in/output and a series resistor. With the standardized operating parameters of 8.2 V and an electrical current value for the ON state from 2.2 mA to 8 mA and an electrical current value for the OFF state from 0.35 mA to 1.2 mA, there results typical values of 1 kOhm for the series resistor and 11 kOhm for the parallel resistor. If the actual electrical current values lie under 0.35 mA, these can be evaluated as indication of a line interruption, while electrical current values over 8 mA are an indication of a short circuit.
In order to be able to cover different applications of the device, a switching amplifier according to EN 60947-5-6 should be able to be used in NAMUR and in non-NAMUR operation. In this regard, different solutions are known from the state of the art. In the simplest and usual form, jumpers are provided, with which the series resistor can be shunted and the electrical current path via the parallel resistor interrupted. In NAMUR operation, the jumper across the series resistor is opened and the jumper in the electrical current path of the parallel resistor closed, while, in the case of non-NAMUR operation, the opposite holds. Thus, each device comes with just one jumper, which can be plugged in at the appropriate location according to application. Disadvantageous in the case of this solution is that, for each change of the operating manner, a service technician must visit the device and tinker with it. This can be very cumbersome in the case of widely distributed devices in large production plants or in the case of devices at exposed locations. For switching the operating manner, the device housing must opened and electronic assemblies contacted and even removed. There is the danger of damage to the device electronics as a result of electrostatic discharges or as a result of locations which are not adequately closed upon the resealing of the housing. Both can lead to malfunction or failure of the device. Furthermore, the operating electronics cannot register the operating state and therewith a self-diagnosis is impossible. Likewise, a remote-controlled switching of the operating manner is not possible.
It is, consequently, also known to replace the jumper with relays. These have, however, the disadvantage of additional space requirement, increase the power consumption of the device and therewith also the thermal load on the device electronics and are due to the movable components relatively sensitive to vibrations. It is, consequently, also known to use, instead of relays, semiconductor switches, which especially do not have the vibration sensitivity. They are, however, relatively expensive.
The application of a 4-20 mA NE43 interface, such as above described, is not possible in connection with an interface meeting EN 60947-5-6, since no sufficiently reliable differentiating of the ON/OFF states of the EN 60947-5-6 output signal is possible.
An object of the invention, therefore, is to provide an improved measuring device having a configurable measuring and operating electronics for delivering a measurement signal.